1. Field of the Invention
The present invention relates to an anti-skid brake control system (generally abbreviated to an "ABS") for use in four-wheel drive vehicles, which system is capable of preventing wheel lock-up and thus providing maximum effective braking, and specifically to an anti-skid brake control system for a four-wheel drive vehicle, which system is correlated with an effective wheel-speed sensor arrangement advantageous to both a four-wheel drive vehicle not equipped with an ABS and a four-wheel drive vehicle equipped with an ABS.
2. Description of the Prior Art
On four-wheel drive vehicles with an ABS, for the purpose of ensuring a more precise anti-skid brake control, four wheel-speed sensors are traditionally mounted on respective road wheels. Alternatively, three wheel-speed sensors may be installed to assure an efficient anti-skid brake control according to which a front-left road wheel, a front-right road wheel and a rear road wheel are independently controlled with each other. The latter anti-skid brake control system with three wheel-speed sensors, called a three-channel, three-sensor ABS, has been disclosed in Japanese Patent Provisional Publication (Tokkai Heisei) No. 3-246159. As is generally known, during braking, more of the car weight is transferred to the front road wheels, and thus the wheel load exerted at the front wheels becomes greater than the rear wheels. It will be appreciated that a more precise anti-skid brake control is required at the front wheel side, whereas a very precise anti-skid brake control is not required at the rear wheel side. In the above-noted conventional three-channel anti-skid brake control system disclosed in the Japanese Patent Provisional Publication No. 3-246159, three wheel-speed sensors, consisting of revolution-speed sensors, are attached respectively to a front-left road wheel, a front-right road wheel and a rear propeller shaft connected to a rear differential. Such a three-channel anti-skid brake control is made to independently control the respective road wheels, so that the front-left road wheel is controlled on the basis of a signal from the front-left road wheel speed sensor, the front-right road wheel is controlled on the basis of a signal from the front-right road wheel speed sensor, and the rear road wheels are commonly controlled on the basis of a signal from the propeller-shaft revolution-speed sensor. The value of the signal from the propeller-shaft rotational sensor essentially corresponds to a mean value of the rear-left wheel speed and the rear-right wheel speed. In recent years, the four-wheel drive vehicle with the ABS is constructed to control a driving-torque distribution between the front wheels and the rear wheels, by adjusting an engaging force of a transfer clutch. The driving-torque distribution control is important for the purpose of switching from a two-wheel drive mode to a four-wheel drive mode, or vice versa. Conventionally, such a driving-torque distribution control requires only a mean value of the front-left and front-right wheel speeds and a mean value of the rear-left and rear-right wheel speeds, but never requires all wheel speeds (revolution speeds at respective road wheels). Actually, a revolution speed (essentially corresponding to a mean wheel speed at the front wheel side) of a front propeller shaft connected to a front differential and a revolution speed (essentially corresponding to a mean wheel speed at the rear wheel side) of a rear propeller shaft connected to a rear differential are utilized for the driving-torque distribution control. In case of four-wheel drive vehicles not having option setting for an ABS, it may be preferable to mount a revolution-speed sensor on a front propeller shaft and a revolution-speed sensor on a rear propeller shaft.
However, in case of prior-art four-wheel drive vehicles having option setting for an ABS, as seen in FIG. 14, four revolution-speed sensors 39FL, 39FR, 39RL and 39RR are often pre-mounted on respective four road wheels 12FL, 12FR, 12RL and 12RR of each vehicle, in consideration of an additional installation of an ABS on the four-wheel drive vehicle not equipped with the ABS. In the prior-art four-wheel drive vehicle with the ABS as shown in FIG. 14, an anti-skid brake control section 18b, which will be abbreviated to an "ABS control section", can achieve a high-accuracy, four-channel anti-skid brake control on the basis of signals from the respective sensors 39FL to 39RR. The signal from each revolution-speed sensor is generally produced in the form of a pulse signal. On the other hand, a torque-distribution control section 18a, which will be hereinafter abbreviated to a "4WD control section", receives a front-wheel mean revolution-speed indicative signal N.sub.F (based on the two pulse signals P.sub.FL and P.sub.FR) and a rear-wheel mean revolution-speed indicative signal N.sub.R (based on the two pulse signals P.sub.RL and P.sub.RR) from the ABS control section 18b, and executes a driving-torque distribution control on the basis of the two signals N.sub.F and N.sub.R. For instance, when the mean revolution speed indicative signal value NR of the rear wheel side becomes greater than the mean revolution speed indicative signal value N.sub.F of the front wheel side, a driving torque (power), which is produced by an engine 10 and transmitted through a transmission 20, will be delivered to a front differential 26 through a front propeller shaft 24 as well as to a rear differential 30 through a rear propeller shaft 32, by increasing an engaging force of a transfer clutch 66 operably accommodated in a transfer 22. In this case, four revolution-speed sensors would be mounted on respective road wheels irrespective of a four-wheel drive vehicle equipped with an ABS or a four-wheel drive vehicle not equipped with an ABS. Such installation of four sensors is expensive. Additionally, the installation of four sensors is uneconomical in case of a four-wheel drive vehicle not equipped with an ABS, for the reasons set out above.
In case of the previously-noted anti-skid brake control system disclosed in the Japanese Patent Provisional Publication No. 3-246159, in which the front-left wheel speed (the front-left wheel revolution-speed), the front-right wheel speed (the front-right wheel revolution-speed) and the revolution speed of the rear propeller shaft are detected to provide a three-channel anti-skid brake control, the signals from the three sensors is simultaneously utilized for the driving-torque distribution control executed by the 4WD control section. That is, the revolution speed detected at the rear propeller shaft is regarded directly as a mean wheel speed of the rear wheels, while a mean front-wheel speed is derived as a mean value of the detected front-left wheel speed and the front-right wheel speed. The sensor arrangement described in the Japanese Patent provisional Publication No. 3-246159 is superior to that shown in FIG. 14. However, in case that the user selects a four-wheel drive vehicle not equipped with an ABS, it is still uneconomical that three revolution-speed sensors are provided on the four-wheel drive vehicle, because it is desired to install only two revolution-speed sensors respectively at front and rear propeller shafts to reduce manufacturing costs of the vehicle, while ensuring a driving-torque distribution control.